Originally Posted by SchrodingersCat
Could even take that in the other direction: girl-on-girl action stimulates men and lubricates women? Increases the probability of coitus, and of that coitus leading to reproduction: another evolutionary advantage.
According to Sex at Dawn, one of the best ways to increase sperm production/sexual desire in males is more akin to what we would colloquially call Hotwife or cuckoldry.
I understand your idea it is very interesting from an evolutionary standpoint but I do think it is interesting that the most profitable way to increase sperm is the most culturally fought against even amongst Poly's. The strength of the OPP probably elevates girl on girl sex as being the most desirable for male observers but actually men get more turned on by MF sex but tend to be disgusted by it wrt their own partners due to cultural sexual jealousy, GonG is seen as less of a threat to the man.